The measurement of ultralow interfacial tension is of intrinsic interest in oil-water systems. Interfacial tensions in such system are relevant to the understanding of various phenomena, e.g., the thermodynamics of formation of microemulsions and the mechanism of tertiary oil recovery such as micellar flooding and interfacial tension flooding. It is known that the tertiary oil recovery technique can enhance the oil recovery up to about 80% of all the oil reserve in the oil reservoir. The interfacial tension of an oil-water system is about 50 mN/m which is decreased to a level of 10.sup.-2 mN/m or even to a ultralow level of 10.sup.-4 .about.10.sup.-5 mN/m after an appropriate surfactant being added to the oil-water system. There are various methods for measuring interfacial tension of a two-phase fluid system known in the art, for example capillary rise method, pendant drop method, spinning drop method, light scattering method, drop weight method, drop volume method, maximum bubble pressure method, Wilheimy plate method, DuNouy ring method, and oscillating jet method, etc. Most of these methods are only suitable for measuring interfacial tension higher than 1 mN/m, and only a few techniques are available for measuring ultralow interfacial tension (IFT). They are spinning drop, laser light scattering, and droplet deformation methods.
For the pendant drop method, the pendant drop created has to have an equator, i.e. the interfacial tension measured is limited to a value higher than 1 mN/m. Recently, Satherley et al. [J. Colloid Interf. Sci., 136, 574 (1990)] utilized the inflection plane of the pendant drop method to measure ultralow IFT and found that the method is suitable for pendant drops without an equator but still having an inflection plane.